In the technique of uranium enrichment by photoexciting and ionizing a selected isotopic component of a vapor, typically generated by heating or electron beam techniques, it has been suggested that the selectively ionized particles are preferably collected from the ionized vapor or plasma by crossed-field, magnetohydrodynamic, effects. Such a system is disclosed, for example, in U.S. Pat. No. 3,939,345, assigned to the same assignee as the present application and incorporated herein by reference. The technique there shown employs the application of a pulsed electric field to the plasma in the presence of an orthogonally directed magnetic field resulting in the acceleration of plasma ions onto trajectories which permit their separate collection apart from the plasma. The electric field is typically applied between a set of plates forming a plurality of U-shaped channel members acting as a cathode with an anode placed centrally within each chamber. A vapor of uranium, including the isotopes to be separated, is then directed into the chambers where they are selectively ionized by isotope type and subjected to the accelerating forces.
The vapor source, typically an electron beam source or induction oven acts as an efficient supplier of electrons to the plasma. Electrons are desired in order to permit the action of the magnetohydrodynamic acceleration forces to direct the ionized particles for collection onto the plates of the cathode. While an additional, filamentary source of electrons may be provided in the plasma the vapor source typically functions as a dominant source of electrons. Because the electrons travel with the vapor, from the point of generation out to the chambers where selective ionization and extraction is achieved, there is a significant probability of interacting with vapor particles resulting in excitation of the vapor particles from the ground or other low-lying energy states. Accordingly, a significant number of the particles in the vapor reaching the chambers are no longer in the ground or other very low-lying energy states to which excitation radiation in the form of laser beams is tuned for isotopically selective photoexcitation. As a result, the portion of those energized particles which are of the desired isotope are not available for photoexcitation and ultimate collection on the plates as enriched fraction. The unavailability of the significant number of these particles serves to reduce the efficiency of the process beyond what might otherwise be achieved.
Additionally, the presence of numbers of energetic electrons increases the chance of de-excitation collisions with excited state particles, reducing the excited state life time and lowering system enrichment efficiency.